Characterization of the domains of the erythropoietin receptor necessary for induction of cell growth and differentiation

Low doses of EPO activate MAP kinases but not JAK2-STAT5 in rat vascular smooth muscle cells

Previous reports have shown a direct effect of erythropoietin (Epo) on vascular smooth muscle cells (VSMCs). Our aim was to assess expression of the Epo receptor (EpoR) on VSMCs and to study the activation of two major signaling cascades activated by Epo, namely JAK2/STAT5 and MAPK pathways. All experiments were performed in parallel using the Epo-responsive UT7 cell line. From semiquantitative RT-PCR experiments, VSMCs were estimated to express approximately 30-fold less EpoR mRNA than UT7 cells. Epo-induced phosphorylation of proteins involved in the EpoR/JAK2/STAT5 cascade could not be detected in VSMCs, even using pharmacological doses of Epo (250 IU/ml). In contrast, a strong activation of MAP kinase pathway was detected with as low as 10 IU/ml Epo. We suggest that MAPK activation reflects a physiologically relevant effect of Epo on VSMCs that may be correlated to cell proliferation.

A minimal cytoplasmic subdomain of the erythropoietin receptor mediates p70 S6 kinase phosphorylation

Erythropoietin (EPO) is a lineage-restricted growth factor that is required for erythroid proliferation and differentiation. EPO stimulates the phosphorylation and activation of p70 S6 kinase (p70 S6K), which is required for cell cycle progression. Here, the minimal cytoplasmic domains of the EPO receptor (EPO-R) required for p70 S6K activation were determined.Ba/F3 cells were stably transfected with wild-type (WT) EPO-R or EPO-R carboxyl-terminal deletion mutants, designated by the number of amino acids deleted from the cytoplasmic tail (-99, -131, -221). Transfected cells were growth factor deprived and then stimulated with EPO. p70 S6K, JAK2, IRS-2, and ERK1/2 phosphorylation/activation were examined. The ability of transfected 3-phosphoinositide-dependent protein kinase 1 (PDK1) to reconstitute p70 S6K phosphorylation in EPO-R mutants also was determined. Phosphorylation and activation of p70 S6K, JAK2, IRS-2, and ERK1/2 in Ba/F3 cells transfected with EPO-R-99 or EPO-R-99Y343F were similar to WT EPO-R. In contrast, EPO-dependent p70 S6K phosphorylation/activation, as well as IRS-2 and ERK1/2 phosphorylation, were minimal or absent in cells transfected with EPO-R-131 or EPO-R-221. JAK2 phosphorylation was reduced significantly in cells transfected with EPO-R-131 and abolished with EPO-R-221. To examine the role of PDK1, a kinase known to phosphorylate p70 S6K, Ba/F3 EPO-R-131 cells were transiently transfected with PDK1. WT constitutively active PDK1 restored p70 S6K phosphorylation in Ba/F3 EPO-R-131 cells but not in Ba/F3 EPO-R-221 cells. The results demonstrate that a minimal cytoplasmic subdomain of the EPO-R extending between -99 and -131 is required for p70 S6K phosphorylation and activation. The results also demonstrate that PDK1 is a critical component in this signaling pathway, which requires the presence of domains between -131 and -221 for its activation of p70 S6K.

Coinduction of Embryonic and Adult-Type Globin mRNAs by Sodium Butyrate and Trichostatin A in Two Murine Interleukin-3–Dependent Bone Marrow–Derived Cell Lines

Using an RNase protection assay, globin mRNA species expressed in clones derived from Ba/F3 and B6SUtA cells transfected with the erythropoietin receptor (EpoR) and selected with erythropoietin (Epo) were compared with globin mRNA species induced in corresponding parental cells by sodium butyrate (SB) and trichostatin A (TSA). βMajor/βminor- and -1/-2–globin mRNAs were the major species, with trace amounts of ɛ-globin mRNA, formed in Epo-stimulated EpoR+ Ba/F3 clones, whereas SB and TSA allowed expression of all species of globin mRNAs, ie, ɛ, βh1, βmajor/βminor, ζ, and -1/-2, in parental Ba/F3 cells. In contrast, ɛ- and -1/-2–globin mRNAs were the major species present in Epo-stimulated EpoR+ B6SUtA clones, whereas SB and TSA activated ɛ-, βh1-, βS/βT-, and -1/-2–globin genes in parental B6SUtA cells; ζ-globin mRNA was not detected in SB- and TSA-treated B6SUtA cells. Because TSA is a specific inhibitor of histone deacetylase, the mimicry of action exhibited by SB and TSA suggests that the effects of SB are mediated through its ability to inhibit histone deacetylase and that histone deacetylase is an integral part of the repression of globin genes in these interleukin-3–dependent cells. Efficient coinduction of embryonic and adult types of globin mRNA in bone marrow cell lines derived from adult mice indicates that adult hematopoietic precursors possess an embryonic nature. These cell lines are useful models to study the mechanism(s) of developmental globin gene switching.

Coinduction of Embryonic and Adult-Type Globin mRNAs by Sodium Butyrate and Trichostatin A in Two Murine Interleukin-3–Dependent Bone Marrow–Derived Cell Lines

Using an RNase protection assay, globin mRNA species expressed in clones derived from Ba/F3 and B6SUtA cells transfected with the erythropoietin receptor (EpoR) and selected with erythropoietin (Epo) were compared with globin mRNA species induced in corresponding parental cells by sodium butyrate (SB) and trichostatin A (TSA). βMajor/βminor- and -1/-2–globin mRNAs were the major species, with trace amounts of ɛ-globin mRNA, formed in Epo-stimulated EpoR+ Ba/F3 clones, whereas SB and TSA allowed expression of all species of globin mRNAs, ie, ɛ, βh1, βmajor/βminor, ζ, and -1/-2, in parental Ba/F3 cells. In contrast, ɛ- and -1/-2–globin mRNAs were the major species present in Epo-stimulated EpoR+ B6SUtA clones, whereas SB and TSA activated ɛ-, βh1-, βS/βT-, and -1/-2–globin genes in parental B6SUtA cells; ζ-globin mRNA was not detected in SB- and TSA-treated B6SUtA cells. Because TSA is a specific inhibitor of histone deacetylase, the mimicry of action exhibited by SB and TSA suggests that the effects of SB are mediated through its ability to inhibit histone deacetylase and that histone deacetylase is an integral part of the repression of globin genes in these interleukin-3–dependent cells. Efficient coinduction of embryonic and adult types of globin mRNA in bone marrow cell lines derived from adult mice indicates that adult hematopoietic precursors possess an embryonic nature. These cell lines are useful models to study the mechanism(s) of developmental globin gene switching.